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PCB Tech
PCB Maintenance Skills Examples Learning Purpose
PCB Tech
PCB Maintenance Skills Examples Learning Purpose

PCB Maintenance Skills Examples Learning Purpose


1. PCB maintenance example

    important hint:

When the chip pins are short-circuited to ground or power, the short-circuit resistance value is between 10-25 milliohms.

A one-inch-long attached copper wire has a resistance of approximately 40-50 milliohms.

For example: when a 74640 performs an online function test, its pin 6 shows that the resistance to ground is 1 ohm, and the resistance to ground on the 6th pin measured with QT50 is about 160 milliohms. It is suspected that it is a short-circuit fault caused by other devices connected to it. After QT50 inspection, it is found that the chip is connected with a resistor bank and a setting switch. The resistance to ground measured at the switch contact is about 40 milliohms, which is lower than the resistance measured on pin 6, so it means that the short-circuit fault of pin 6 to ground displayed by the chip is because the setting switch is connected to ground. It is not a malfunction of the chip itself.

Under normal circumstances, the pins of the bus device have no (or very little) grounding and power supply in the design. If there is a pin grounding or power supply, please test the device again.

In the absence of a good board for reference and comparison, the bad board can also be repaired by analyzing the pin status and the actual measured waveform. For example, many chips only use part of the logic cells in the circuit design, and the input pins of the remaining unused parts are generally grounded to prevent this part from being in a random operation state and causing interference to the circuit. Analyzing the actual output waveform according to the logic function of the chip will be very helpful to judge whether the tested chip is really damaged.

pcb board

Example 1: 74123 (monostable resonator) pin appears floating (FLT)

In the online function test, the input pin of the device is generally displayed as a high-impedance state (resistance value greater than 1 megaohm). When the device is tested offline, this result will appear if the TTL or CMOS load is not connected. In online testing, the input pin of the chip is usually connected to the output pin of another chip. In order to ensure that the output pin of the chip drives the fan-out load, it is usually low impedance.

In the online test, if an input pin displays "FLT" in the pin status window, it means that the pin is in the floating state, and the pin may be connected to the boundary connection end of the circuit board or tri-state device, or it is connected to the PCB circuit board. open circuit.

By comparing with the state of other input pins, it can be judged whether the state of this pin is normal.

In this example, the 6th pin is connected to the input terminal of the RC circuit. The capacitor in the circuit is charged through a resistor and then discharged via the 6th pin of the chip. Then the input pin cannot be in a high-impedance state, because if it is high-impedance, It cannot discharge the capacitor. A test error occurred when ICFT was performed on the chip. The 7th pin displayed "FLT", and the other same input pin (pin 15) displayed a normal logic level (the impedance to ground is about 550 ohms). Although the test result of QT200 on the chip is "test failure", the output pin of the chip is flipped, so it seems to be a timing problem. If the user is not paying attention, the test result will be ignored and considered as a timing issue.

The above analysis shows that the user carefully observes and analyzes the pin status information, which is extremely important for judging the true fault point. If the input impedance of an input pin is 550 ohms, it will not be in a floating state (FLT). The actual cause of the fault in this example is that the monostable resonator cannot discharge the capacitor normally due to its function damage.

Similarly, the output pin of the device cannot be in a floating state (FLT), because if it is in a floating state, the pin cannot absorb or discharge current, and cannot drive any fan-out load. In addition, users should also pay attention: the impedance of any node to ground cannot be less than 5-10 ohms (unless the node is really short-circuited to the ground-the impedance is about 2 ohms at this time). Commonly used buffer drivers have an impedance of approximately 15-17 ohms in a logic low state.

Example 2: The power supply fixture of the tester cannot provide test power due to poor contact

When power is supplied to the board under test through the power supply fixture of the tester and a chip on the board under test, it is sometimes found that the measured voltage of the power supply pin of the chip under test is only 4.5V, and the test results at this time are often unstable. The reason for this situation may be due to the oxidation of the chip pins so that the power supply fixture of the tester cannot make good contact with the chip pins. When this kind of problem occurs, the user can take different methods to solve it. The most effective method is to connect the power supply terminal of the tester to the power supply cable of the tester through the boundary of the board under test.

Example 3: A certain pin of the chip is short-circuited to ground when the board under test is powered on

This phenomenon is difficult to solve. The test result shows that the impedance of the output pin of the tested chip is 10 ohms (low impedance state), and there is no flip action. The impedance is smaller than the impedance of the normal buffer driver output pin when it is logic low. When disconnecting the power of the board under test, use a three-meter to measure that the pin is not short-circuited to the ground (resistance is greater than 1 kiloohm).

This short-circuit phenomenon occurs only when the board under test is powered on, which may be the cause of the output end of the device under test, or the cause of the input end of the device connected to the output end. The output end of the device under test should be able to absorb current when the logic is low, and when the logic is high, it outputs current to the input pin of the fan-out device.

Example 4: When the board under test is not powered on, one of the pins of the three chips are all short-circuited to ground

If the device under test is internally short-circuited to ground when it is powered on, there will be no short-circuit to ground in the measurement when no power is applied. In this case, a milliohmmeter can be used to measure the resistance of the output pin of the device under test to ground and the resistance of the input pin of the connected fan-out device to ground. The measurement point with the smallest resistance is the real damaged device.

The specific phenomenon is that there are three chips on the tested board that have "test failure" during ICFT, and the second pin of the bus device 74374, the 18th pin of 74244 and the other 74244 pin are displayed in the pin status window. Pin 3 is short-circuited to ground respectively.

First find out the chip closest to the PCB short-circuit point. When using the QT50 short-circuit tracker for testing, set the measurement range to 200 milliohms, measure the resistance of the short-circuit pins of the three chips to the ground, and find the pin with the lowest resistance. The method is: connect one probe to the power supply ground of the board under test, and connect the other probe to the second pin of 74374, the measured resistance is 160 milliohms, and then connect to the 18th pin of 74244, and measure The resistance is 90 milliohms, and then connected to the third pin of another 74244, and the measured resistance is 10 milliohms. Then this pin is the short-circuit point that caused the bus error.

The following question is to determine whether the real short-circuit point is inside 74244 or on the copper wire attached to the outside of the PCB short-circuit pin. The method is: use one probe of QT50 to ground, and connect the other probe to the part above the solder joint of the third pin of 74244, read the resistance of this pin to ground (about 10 milliohms), and then Connect the probe pen to the attached copper wire 3-4 mm outside the solder joint of the 3rd pin, and then read the resistance value of the 3rd pin to the ground at this time (about 6 milliohms). This result shows that the real short-circuit point is the external copper wire attached to pin 3.